1893.] Nervous System on the Cardiac Rhythm, §c. 471 



III. On the Effects of Excitation of Affeeent Nerves upon the 



Cardiac Rhythm. 



The changes excited in the cardiac rhythm as a result of stimula- 

 tion of afferent nerves in favourable circumstances may be either 

 of the nature (A) of a slowing or (B) an acceleration of the heart 

 beat; whether the former (A) or the latter (B) takes place varies 

 according to the conditions present at the time — the particular nerve 

 stimulated, the strength and suddenness of the stimulation, the exact 

 degree of chloroform anaesthesia, and the state of the medullary 

 centres in other respects. 



Some Features in the Results of Stimulation of Afferent Nerves. 



1. Speaking generally, excitation of visceral or splanchnic afferent 

 nerves {e.g., vagus, cervical sympathetic, abdominal splanchnic, &c), 

 is more readily effective in altering the cardiac rhythm under chloro- 

 form than excitation of somatic afferent nerves. The result may be 

 acceleration or slowing, or an alternation of these, in the case of 

 either class of nerves. 



2. Moreover, there is a very important difference between the action 

 of the splanchnic and that of the somatic afferent nerves upon the 

 pulse rate. The acceleration resulting from a stimulation of somatic 

 nerves is accompanied by signs of diffuse motor excitation as indi- 

 cated by the occurrence of more or less general muscular contrac- 

 tion, whereas acceleration often results from stimulating splanchnic 

 or visceral nerves when there are no concomitant movements and no 

 sign of any association with general motor excitation at all. In 

 curarised animals, such as have been used by the great majority of 

 investigators, this difference vrould not of course be evident. 



3. Again, strong sudden stimulation of an afferent nerve may 

 cause marked slowing, while weaker and more gradual stimulation of 

 the same nerve causes acceleration. Cceteris paribus, strong sudden 

 stimulation is relatively more apt to cause slowing. (Of course this 

 does not necessarily hold good with natural stimulation of various 

 afferent nerves.) 



4. Further stimulation of such nerves (e.g., the brachial or inter- 

 costal) may cause notable slowing of the heart in suitable conditions 

 of anaesthesia (in the period already referred to as the most favour- 

 able one) ; but they commonly cause acceleration as their only effect 

 on the cardiac rhythm when the anaesthesia is rendered somewhat 

 deeper ; and at a still later phase the afferent excitation induces no 

 change at all in the pulse rate. An excitation which causes slowing, 

 followed by acceleration, of the heart, at a certain phase of anaesthesia, 

 commonly fails to cause any slowing at all, but only acceleration — if 

 any effect at all — when more chloroform has been given. 



